<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>没有宽度和高度声明实现的全屏自适应效果</title>
<style>
html, body { height: 100%; }
.overlay {
	position: absolute;
	left: 0; top: 0; right: 0; bottom: 0;
	background-color: #000;
	opacity: .5; filter: alpha(opacity=50);
}
</style>
</head>

<body>
<div class="overlay"></div>
</body>
</html>